Plastic reprocessing with controlled decontamination

ABSTRACT

A device for reprocessing used plastic containers including a system for analyzing the degree of contamination of the plastic, a system for determining decontamination process parameters as a function of the degree of contamination thus detected, and a system for controlled decontamination of the plastic according to the decontamination process parameters thus determined. The system for determining decontamination process parameters provides determined decontamination process parameters to corresponding decontamination control elements that are automatically adjusted depending on the degree of contamination.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of the U.S. national stage under 35U.S.C. §371, of international application no. PCT/EP2004/01 1230, havingan international filing date of Oct. 8, 2004, and claims priority toGerman application no. 103 48 145.1 filed on Oct. 13, 2003.

FIELD OF THE DISCLOSURE

The disclosure relates to a method and a device for reprocessing usedplastic containers, especially PET bottles.

BACKGROUND OF THE DISCLOSURE

Methods and devices for reprocessing used plastic containers are known.In most cases the labels are released and removed as the first step.Then in a next step a mill is used to pulverize the containers toflakes. The resulting mixture is washed and cleaned to remove anyremaining residues of glue. Then the flakes are separated according todifferent types of plastic, e.g. PET and polyethylene.

Next, the flakes are cleaned in a decontamination step so that they canbe reused to produce new plastic containers.

Thus for example, a decontamination process in which PET flakes frompulverized beverage bottles are subjected to a washing treatment in awasher is known from DE-10002682. Furthermore U.S. Pat. No. 5,688,693describes a method in which highly contaminated bottles or flakes areidentified and removed from the reprocessing operation.

However, the known processes are based on the problem that thedecontamination process step of the reprocessing operation is adjustedto worst case conditions which means that the cleaning process is alwaysperformed, regardless of the degree of soiling of the starting material,in such a way that even the most contaminated plastic bottles and/orplastic flakes are adequately cleaned. Therefore, these known methodsare not economical, even when an adequate cleaning is achieved.

SUMMARY OF THE DISCLOSURE

The object of the present disclosure is thus to provide a method and adevice which will increase the profitability of reprocessing of usedplastic containers, especially PET bottles, and will allow the requireddecontamination process step to be carried out under improvedconditions.

According to this disclosure, the thod thus includes the followingsteps: a) analyzing the degree of contamination of the plastic, b)determining decontamination process parameters as a tinction of thedegree of contamination found in step a), and c) controlleddecontamination of the plastic according to the decontamination processparameters thus determined.

With this method, the decontamination step is automatically adapted tothe actual contamination of the plastic, thanks to the determination ofdegree of contamination. By controlled decontamination, excess cleaningis thus prevented, and this yields a reprocessing operation that can becarried out more economically.

In a preferred embodiment, impurities present in the plastic and therespective concentration thereof can be ascertained in step a) of apreferred embodiment. With a contamination profile broken down in thisway, it is possible to ascertain how the plastic is contaminated and howgreat the respective contamination is. Contaminants, i.e., impurities,are understood to refer to both substances that are hazardous to healthand flavorings that would have harmful effects on the subsequent use ofthe recycled plastic containers when present even in small quantitiesdue to their low perception threshold.

The contaminants detected may advantageously be combined intocontaminant groups. The contaminants thus detected and theconcentrations thereof are used in step b) to determine thedecontamination process parameters. A determination as a function of allthe contaminants detected may be very time-consuming and would thus leadto complicated control algorithms. It is therefore advantageous tocombine individual components having similar properties. For example,hydrocarbons or several hydrocarbon subgroups having certain molecularweight ranges that are determined in advance could be combined here. Itwould also be conceivable to group the impurities according to physicalproperties, e.g., according to their diffusion constants.

In a preferred embodiment, a process temperature adapted to the degreeof contamination may be determined as a decontamination processparameter in step b). Since the contaminants are usually not only at thesurface of the plastic material but also in interior of the material,they must first diffuse to the surface, from which they can then heremoved by washing, for example. According to the laws of diffusion,diffusion coefficients are a function of temperature, so it is possibleto accelerate the decontamination process by adjusting the temperaturefor the prevailing level of contamination. For example, a higher processtemperature will be selected for highly contaminated plastics than forless contaminated plastics.

In an especially advantageous embodiment of the method, a processduration adapted to the degree of contamination may be determined as adecontamination process parameter in step b). According to the diffusionlaw, the pulp flow density is a function not only of temperature butalso of time, so the decontamination step may thus also be optimized byadjusting the retention time. For example, at a uniform processtemperature, less contaminated plastic can be decontaminated morerapidly than highly contaminated plastic which should remain in thedecontamination process for a longer period of time. Thus, adecontamination level which is sufficient to comply with food standardscan be achieved at the end of the decontamination process for bothdegrees of contamination.

The degree of contamination of the plastic can conceivably be determinedin step b) by adding up the concentrations of the contaminants orcontaminant groups thus detected. The decontamination process parameterscan be determined easily and thus quickly by such an estimation of thetotal contamination, with the decontamination being continued as afunction of a degree of contamination that has been determined.

A weighting factor may expediently be assigned to the individualcontaminants or contaminant groups as a function of an intensity ofcontamination that corresponds to the contaminant or contaminant groupand then the degree of contamination is determined from the weightedsummation of the contaminations of the contaminants and contaminantgroups thereby detected. Through such a weighted summation, it ispossible to take into account the fact that different contaminants causedifferent degrees of contamination. For example, in the case offlavorings, the perception threshold is relatively low, especially forlemons. Due to a high weighting factor for lemons, it is possible totake into account this substance in the determination of thedecontamination process parameters in comparison with other substances.

In another embodiment, the decontamination parameters may be determinedin step b) as a function of the concentrations of a predetermined numberof contaminants or contaminant groups. Thus, for example, the optimizeddecontamination process parameters can be determined only on the basisof the ten contaminants occurring most commonly or only on the basis ofthe total hydrocarbons. For example, if the reprocessed plastic is notto be reused in the food industry, then contamination due to flavoringswill play only a subordinate role and thus need not be taken intoaccount in the determination of decontamination parameters. This makesis possible to adapt the process even better o the requirements of thematerial to be reprocessed.

In one variant of the invention, the decontamination process parameterscan be determined independently of one another in step b) for at leasttwo, especially for all the contaminants detected or contaminant groupsdetected, and in step c) the decontamination process parameters forwhich the profile of requirements is most stringent are used. Under theassumption that the decontamination takes place for the individualcontaminants independently of one another, it is thus possible to assurein a simple manner that the decontamination process parameters selectedfor a given contamination profile are those for which adequatedecontamination can be ensured with respect to all the contaminantsdetected.

In an especially advantageous embodiment, the decontamination parameterscan be determined as a function of controllable threshold values in stepb). Thus, depending on the desired type of recycling, thedecontamination process can be adjusted in such a way that it ispossible to ensure with sufficient accuracy that the purity required forthat type of recycling is maintained.

Step c) can preferably be performed only when the degree ofcontamination exceeds a predetermined first threshold value. If theanalysis shows that the degree of contamination of the plastic is so lowthat the reprocessing operation could be continued even withoutdecontamination, then it is possible, thanks to such a predeterminedthreshold value, to skip this specific decontamination step, which wouldfurther accelerate the process and thus make it more economical.

Preferably, the plastic can be re-shredded between process steps b) andc) if it is found that the degree of contamination exceeds apredetermined second threshold value. Re-shredding reduces the length ofthe diffusion path and thus also reduces the time required to bring thecontaminants to the surface of the plastic. Therefore, the desireddegree of purity can be achieved in a shorter period of time, bycomparison, in a case of severe contamination in which adequate successwould otherwise be achieved only by an excessively long decontaminationtime. The second threshold value is preferably higher than the firstthreshold value.

In a preferred embodiment, if the degree of contamination exceeds apredetermined third threshold value, then the plastic can be sorted outand removed from the reprocessing operation instead of going throughsteps b) and c). This third threshold value is selected so that anyplastic having a contamination level so high that decontamination is nolonger economically feasible will be removed from the process and notrecycled. This further contributes to the economic feasibility of theprocess. The third threshold value here is advantageously higher thanthe first and third threshold values.

The decontamination process parameters can advantageously be determinedin step b) with the help of a numerical diffusion model, where thedegree of contamination is a parameter of the model. With the laws ofdiffusion and the known diffusion coefficients of the contaminantsand/or contaminant groups, the optimized temperature and/or duration ofthe decontamination process can be determined again continuously on thebasis of the measured degree of contamination.

Fortunately in step b), the decontamination process parameters can alsobe determined by comparing the degree of contamination with apredetermined data record. Such a data record can be compiled eitherexperimentally or through model calculations. Depending on theconcentrations and/or the presence of contaminants, it is thus possibleto determine the parameters that fit the respective situation byadjusting the measured values with values saved in a database.

In an especially preferred embodiment, the plastic may be added to oneof at least two partial quantities, depending on the degree ofcontamination thus determined, between steps a) and b), and in step b)decontamination process parameters are determined for each of the atleast two partial quantities and in step c) decontamination is performedaccording to the decontamination process parameters thus determined foreach of the partial quantities. This permits greater flexibility in theprocess because, for example, plastics with a similar degree ofcontamination are collected in different partial quantities and then thepartial quantities are decontaminated independently of one another. Itis also possible to set up buffer storage where the partial quantitiescan be stored until enough material has been collected, only thencontinuing with the decontamination. This makes it possible to utilizethe process more economically.

According to this disclosure, the device for carrying out the methodincludes a system for analyzing the degree of contamination of theplastic, a system for determining decontamination process parameters asa function of the degree of contamination thus detected and a system forcontrolled decontamination of the plastic according to thedecontamination process parameters thus determined.

With this device, the decontamination can be automatically adjusted tothe actual contamination of the plastic thanks to the determination ofthe degree of contamination. Excessive cleaning is thus prevented by thecontrolled decontamination.

The system for performing the analysis may advantageously include a massspectrometer. A mass spectrometer makes it possible to determine thecontaminants according to type and quantity and are therefore especiallysuitable for the device according to this disclosure.

The mass spectrometer is advantageously configured so that itessentially determines the degree of contamination in real time. Bymeans of a rapid analysis of the degree of contamination of all flakesin conjunction with a rapid determination of the decontamination processparameters, it is thus possible to accelerate the entire process.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present disclosure are depicted in thedrawings and explained in greater detail below; the drawings show:

FIG. 1 a flow chart of a first exemplary embodiment of the disclosedprocess,

FIG. 2 a flow chart of a second embodiment of the disclosed process,

FIG. 3 a schematic view of a first embodiment of the disclosed process,and

FIG. 4 an example of the possible adaptation of the decontaminationprocess parameters as a function of a contamination level detected aswell as the time and the temperature.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1 shows a flow chart of a first embodiment of the inventive methodfor reprocessing used plastic containers, especially PET bottles. Instep 100, used PET bottles are supplied. They are washed and shredded inseveral steps 101 thereafter. Furthermore, the various materials, e.g.,PET, polyethylene of the caps, paper or plastic labels, metal caps andadhesives are separated. The process steps described below pertain onlyto the PET flakes formed by shredding.

In step 102 the degree of contamination of all plastic flakes isanalyzed according to process step a). To do so, for example, a massspectrometer is used, creating a contamination profile based on thecontaminant and amount of contamination. From the contamination profilethereby determined, the degree of contamination is then determined. Thedegree of contamination can be determined in various ways. For example,the contamination level is determined by adding up the concentrations ofthe contaminants detected. In a second variant, a weighted sum may becalculated by assigning a weighting factor to a contaminant depending onthe intensity of contamination with this contaminant, then multiplyingthis weighting factor times the respective concentration and only thenperforming the total addition. This makes it possible to ensure that lowconcentrations of highly contaminated substances will play a greaterrole in determining the degree of contamination. According to anothervariant, several contaminants having similar properties such ashydrocarbons or flavoring agents are combined into groups and a degreeof contamination is determined for each component of this groupaccording to the methods already described.

In step 103, this degree of contamination that was analyzed in step 101is compared with a predetermined threshold value 3 (SW 3) which is madeavailable in step 104. This threshold value 3 may be either fixedlypredetermined or adjusted by the operator of a reprocessing plant inaccordance with circumstances. If the degree of contamination thusdetermined is above this threshold value 3, then the correspondingplastic flakes are sorted out and removed from the process because theyare too highly contaminated. For the case when several degrees ofcontamination have been determined, there is the option of sorting outthe plastic flakes only if all degrees of contamination are above theparticular threshold values 3 or the plastic flakes are sorted out whenthere is at least a degree of contamination above the respectivethreshold value 3. This makes it possible, for example, for PET bottlesto be sorted out even if they have not been contaminated by hydrocarbonsbut instead only traces of flavorings have been found in them.

If the degree of contamination is below threshold value 3, then in step106 the decontamination process parameters corresponding to the degreeof contamination found in step 102 are determined according to processstep b). To determine these parameters, either a numerical model basedon the laws of diffusion is used or the degrees of contamination arecompared with a predetermined data record in step 107. If the numericalmethod is used, then the required duration of the decontamination stepcan be calculated on the basis of the concentrations thus determined ata predetermined decontamination process temperature. Conversely in thecase of a fixedly predetermined decontamination time, the processtemperature that would be required to adequately clean the plasticflakes during this predetermined process time can be calculated. If thecalculation reveals a temperature above a critical transitiontemperature of 220° C., for example, then the process time is lengthenedaccordingly. Instead of calculating the decontamination processparameters anew in each case, in one variant it is also possible todetermine the decontamination process parameters by comparing thedegrees of contamination with a predetermined data record (the latterbeing based on the same laws as the numerical model).

In determination of the process parameters in step 106, a thresholdvalue 1 and a threshold value 2 which are made available in step 108 arealso taken into account. The threshold value 1 indicates for whichdegree of contamination a decontamination is required at allsubsequently. If the contamination is below this limit, the plastic canbe processed further directly without a decontamination step. Thresholdvalue 2 indicates beyond which degree of contamination it isadvantageous to re-shred the plastic to shorten the diffusion paths tothereby accelerate the decontamination. These threshold values may bepredetermined and/or adjusted by the user to thereby adapt the processso that the reprocessed material conforms to quality demands, which mayvary according to the use of the reprocessed material.

Thus if the degree of contamination is below threshold value 1 (step109) then the special decontamination step in the process is skipped andthe reprocessing operation goes directly to performing the IV-increasingstep (IV=intrinsic viscosity) which serves to impart once again to thePET flakes the properties required to be able to manufacture PET bottlesfrom them again (step 113). A certain decontamination also takes placehere because of the heating.

If the degree of contamination is above the threshold value 1, then acheck is performed in the next step 110 to determine whether the degreeof contamination is above the threshold value 2. If this is the case,then in step 111 the plastic flakes are re-shredded and only then sentto the decontamination step 112. If the degree of contamination is belowthe threshold value 2, then the method is continued directly withdecontamination step 112.

Decontamination step 112 is performed with the process parametersdetermined in step 106 and thus represents a controlled process stepaccording to process step c). The final control elements for theindividual process parameters such as temperature, decontamination time,etc., are usually adjusted automatically. Then the plastic flakes aresent to the next process step 113, the IV-increasing step, as describedabove.

Other variants can also be implemented by starting with this firstexemplary embodiment. For example, it is not absolutely necessary toshred the bottles before the contamination analysis and insteadunshredded PET bottles can also be checked for their degree ofcontamination. In another variant, before the contamination analysisstep 102, the PET flakes could be separated, separating PET flakes thatoriginate from thicker bottle head parts from PET flakes from theremaining bottle. This is of interest because the PET flakes of thebottle head are thicker than those from the remainder of the bottle; itis more difficult to clean them because contaminants are situated deeperin the PET material. In the exemplary embodiment described here, theprocess temperature and process time are referred to as decontaminationprocess parameters. However, these are only two examples of processparameters because the cleaning agent used for decontamination mightalso be adjustable.

FIG. 2 shows a second embodiment of the method for reprocessing usedplastic containers. In this figure, process steps 200 through 205 and213 correspond to process steps 100 through 105 and 113 of the firstexemplary embodiment illustrated in FIG. 1. Process steps 200 through205 and 213 have the same features as the corresponding process steps inthe first exemplary embodiment and therefore will not be described indetail below.

The essential difference with respect to the first embodiment is thatafter analyzing the contamination (process step a), several processescan be performed in parallel. For example, if it has been determined instep 203 that the allowed degree of contamination is below the maximumdegree of contamination as defined by the threshold value 3, then adecision is made in step 220 regarding whether the degree ofcontamination is low, moderate or high. Low, moderate and high as usedhere each denote a range of contamination, but the limits between theseranges are predetermined or can be selected freely by the user.

If it has been found that the degree of contamination is low, then nodecontamination is performed as in the first exemplary embodiment andthe method proceeds with the next process step 213, increasing theintrinsic viscosity IV.

If a degree of contamination in the moderate contamination range hasbeen found, then in process step 221 the fitting process parameters Iare determined (process step b) and then the decontamination isperformed in such a manner that it is controlled by the processparameters I thus determined (step 222, process step c). The processparameters were determined exactly as already described in conjunctionwith the first exemplary embodiment. As already described in the firstexemplary embodiment, the final control elements for adjusting thedecontamination process parameters are set automatically according tothe process parameters thus determined. Then the process is continuedwith step 213, increasing the intrinsic viscosity IV.

If it has been found in step 220 that the degree of contamination is inthe range of high contamination, then in process step 223, a second setof process parameters II is determined (process step b) and next thedecontamination is performed in such a manner that it is controlledaccording to these second process parameters II (step 222, process stepc). As already described in the first exemplary embodiment, the finalcontrol elements here are set automatically according to the processparameters thus determined for adjusting the decontamination processparameters. Then the process here again is continued with step 213.

In one variant the three partial quantities are processed further indifferent ways depending on their respective degrees of decontamination.For example, the limit values between the various contamination levelscan be set at low, moderate or high, so that the process is carried outfor the low and moderate partial quantities of decontamination in such away that the PET flakes can then be reused for production of beveragebottles whereas the partial quantity having a high degree ofcontamination—which could not be decontaminated economically to such anextent that it would be acceptable for use in the food industry—isprocessed further (226) for use in a different application where demandsare lower.

In another variant, it is conceivable for the steps 223, 224 and 221,222 not to be performed in parallel but instead for a buffer storage tobe set up, material with the same degree of contamination to be storedtemporarily, and as soon as enough material is in the buffer storage,the respective suitable decontamination for it is performed. Forexample, first one or more batches may be sorted out and thendecontaminated with the partial quantity for which decontamination isachieved most rapidly and then the process is continued with the moreheavily contaminated partial quantity and/or one may wait until thispartial quantity is large enough.

FIG. 3 shows an embodiment of a device for performing the inventivemethod for reprocessing used plastic containers, especially PET bottles.Used PET bottles 301 are charged to a conveyor belt 302. In a firstsection 303, the bottles 301 are washed and shredded. This does not showthe zone in which the different materials are separated from oneanother. Then the prewashed PET flakes 304 emerge from section 303 andare analyzed by a contamination analysis device 305 such as a massspectrometer, for example, in a continuous stream to determine theirdegree of contamination. The analytical device 305 forwards the datathus compiled with regard to the degree of contamination to the controlunit 306 which determines the process parameters corresponding to thedegree of contamination and automatically forwards them to thecorresponding final control elements such as a temperature regulator,the process time setting, etc. in decontamination step 307 of device300. The plastic flakes 304 enter this decontamination section 307 wherethey are cleaned in accordance with their degree of contamination, whichcan take place in a continuous stream or in batches. The PET flakes 304then emerge from the decontamination section 307 and can then be treatedfurther.

It is quite conceivable for the quality of decontamination to be testedafter departure from the decontamination section, and if it is foundthat the decontamination has not been adequate, the process parametersmay then be checked again and adjusted, if necessary, i.e., forming aclosed control circuit with regard to the decontamination parameters.

FIG. 4 shows an example of how the process parameters of temperature andtime can be controlled on the basis of the detected concentration of acontaminant. The logarithm of the concentration of the contaminant isplotted on the Y axis and time is plotted on the X axis. According tothe diffusion law, the logarithm of the concentration decreases linearlywith time at a constant temperature. For example, if a concentration C1is measured and is to be returned to a level C0, then a time t11 will berequired to do so at the temperature T1. At the lower temperature T2, atime t12 would be needed to accomplish the same thing and at an evenlower temperature T3 a time t13 would be needed. If the decontaminationis to be performed in a time period of max, t0, then this can beachieved only at a process temperature T1 in the case of contaminationwith a concentration C1. However, if the detected concentration level isC2, it can be seen that the process temperature is T2 within the timet0. However, if the higher temperature T1 is retained, the desired levelC0 is reached after a time of only t21.

It can thus be recognized on the basis of FIG. 4 that decontaminationcan be optimized by analyzing the degree of contamination anddetermining suitable decontamination process parameters. An upper limitof approximately 230° C. will usually be set for the processtemperature.

1-17. (canceled)
 18. A device for reprocessing used plastic containers,comprising: a system for analyzing a degree of contamination of theplastic, a system for determining decontamination process parameters asa function of the degree of contamination thus detected, wherein aprocess temperature adapted to the degree of contamination is determinedas a decontamination process parameter and/or wherein a process timethat is adapted to the degree of contamination is determined as adecontamination process parameter, and a system for controlleddecontamination of the plastic according to the decontamination processparameters thus determined wherein the system for determiningdecontamination process parameters performs after the system foranalyzing the degree of contamination of the plastic, and wherein thesystem for determining decontamination process parameters providesdetermined decontamination rocess parameters to correspondingdecontamination control elements that are automatically adjusteddepending on the degree of contamination.
 19. The device according toclaim 18, characterized in that the system for performing the analysiscomprises a mass spectrometer.
 20. The device according to claim 19,characterized in that the mass spectrometer is configured so that thedegree of contamination is determined essentially in real time.
 21. Adevice for reprocessing used plastic containers, comprising: a systemfor analyzing a degree of contamination of the plastic; a system fordetermining decontamination process parameters as a function of thedegree of contamination thus detected; wherein a process temperatureadapted to the degree of contamination is determined as adecontamination process parameter, and/or wherein a process time that isadapted to the degree of contamination is determined as adecontamination process parameter; and a system for controlleddecontamination of the plastic according to the decontamination processparameters thus determined, the system for controlled decontaminationincluding one or more decontamination control elements that areautomatically adjusted depending on the actual degree of contaminationof the plastic.
 22. The device of claim 21, wherein the system fordetermining decontamination process parameters performs after the systemfor analyzing the degree of contamination of the plastic.
 23. The deviceof claim 21, wherein the one or more decontamination control elementsincludes one or more of a temperature regulator and a process timedevice.
 24. The device of claim 21, wherein the one or moredecontamination control elements receive determined decontaminationprocess parameters from the system for determining decontaminationprocess parameters such that the one or more decontamination controlelements are automatically adjusted depending upon the degree ofcontamination.